The present invention relates to devices generating radiation in the infrared range (IR) to serve as IR targets for heat seeking detectors, and more particularly to a dynamic IR target wherein dynamic IR images having spatial infrared intensity variations are produced.
IR targets are in use today for such purposes as testing heat seeking missiles and IR tracking systems.
The conventional IR target is the Bly cell, which consists of a very thin membrane, typically of a material such as blackgold, irradiated by a visible image to produce a corresponding infrared image. Incident light energy is converted into heat energy (IR) by absorption. The Bly cell, however, does not have a fast response time or desired dynamic range, and is very fragile due to the thinness of the membrane.
Liquid crystal light valves have been employed in dynamic IR target systems. In the valve, the liquid crystal is backed by a photocell array. Voltages are created across the various portions of the liquid crystal according to the amount of light incident on that portion. Polarized IR radiation from a source illuminating the liquid crystal passes through the liquid crystal, and the plane of polarization of the IR is rotated by different amounts according to the voltage applied across the crystal. A cross polarizer is placed in the path of the IR which passes through the liquid crystal and which is reflected from an IR mirror backing the array. In this way a (polarized) IR image is formed. However, such systems are very expensive and exhibit less than perfect performance in terms of resolution, bandwidth, and response time.
It is understood that researchers at LTV, Inc. have attempted to employ a phase change in vanadium dioxide as a means for providing a dynamic IR target. When the vanadium dioxide is in the conducting phase it reflects IR well, but when it is in an insulating state, it does not reflect well. The phase change occurs close to room temperature. Thus, subjecting the different regions of the vanadium dioxide film to different temperatures results in a mirror for an IR source which has a position dependent reflectivity. In this way an IR image is produced. However, resolution and time response has not proven satisfactory.
It is therefore an object of the present invention to provide a relatively inexpensive, broadband dynamic IR target with a fast response time, good spatial resolution and adequate dynamic range.